1. Field of the Invention
This invention relates to the field of recording and reproducing signals, and particularly to the reduction of distortion of reproduced signals due to zero level offset caused by the overlap of minute areas of recording medium affected by successive signal increments.
2. The Prior Art
It has been the practice in video tape recorders to modulate the frequency of a carrier with the luminance signal and record the resulting frequency-modulated signal without further change, except for limiting and filtering it. The frequency modulated luminance signal causes the recording transducer to produce a magnetic field of extremely limited extent. This field enters the layer of magnetic particles on the tape and realigns the magnetic domains of those particles within a certain distance of the air gap of the transducer, the distance being determined by many factors, including the frequency of the signals applied to the transducer. Lower frequency signals affect the magnetic particles more distant from the air gap and thus farther into the magnetic layer than higher frequency signals.
As the tape (it should be realized that other forms of magnetic recording medium are the equivalent of tape, insofar as this discussion is concerned) moves relative to the transducer, the magnetic fields are spread along the recording path. Since the frequency modulated signal is amplitude-limited, the reversals of polarity of both the signal and the resulting recorded incremental fields are relatively sharp. As a result, the magnetization due to one polarity of field overlaps, to some extent, the magnetization due to the opposite polarity immediately preceding. However, when the applied frequency goes from a relatively lower value to a higher value, the penetration of magnetization into the layer of magnetic particles decreases, so that the effective overlap decreases.
When a tape recorded in the manner just described is played back, flux from the minute magnetic fields of the recorded signal produces a voltage across the coil of the transducer. This may be the same transducer as was used for recording, and hence, in the following specification and claims, the term "recording transducer" is intended to distinquish the transducer from transducers that change one form of energy into another but are not capable of either recording or playing back. Specifically, it is to be understood that a "recording transducer" may also be used as a playback transducer. The voltage induced during playback is a function of the total flux linked at any instant by the transducer and is therefore affected by the amount of overlap on adjacent, incremental magnetic fields. If the amount of overlap shifts suddenly, the effect may be to displace the zero level of the reproduced frequency modulated signal. When this reproduced signal is limited, or clipped, before being demodulated, this displacement of zero level can result in a false output signal.